The present invention relates to a method of producing a fermented milk drink by using lactic acid fermented milk as a raw material, without using any stabilizer such as pectin and gum etc., which drink is adapted for direct drinking (ready-to-drink) and in which the dispersion of milk protein and its beautiful milky white appearance are both stable for a long period.
In general, fermented milk passes across the isoelectric point (pH 4.6) of the milk protein slowly by the action of the lactic acid bacteria, differing from the case of the acid-added milk, and accordingly the milk protein particles become larger and coagulated protein is produced, so that due to these causes etc., the fermented milk becomes very likely to precipitate. Therefore, the production of drinks of such a kind that makes a good use of the taste of fermentation, particularly drinks of such a type that presents a white appearance and is adapted for direct drinking, becomes difficult.
If the milky white appearance is desired, the milk protein particle must be large; but in this case the problem of precipitation is severe. If precipitation is to be avoided by dissolving the milk protein, then the desired milky white appearance is lost. Namely, it can be said that prevention of the precipitation is contrary to the requirement of beautiful milky white appearance.
Accordingly, the production of the fermented milk drink such as the present invention has heretofore been thought very difficult, and, insofar as is known, the prior art never succeeded in attaining an object such as that of the present invention. For example, there is disclosed in Japanese Patent Application Laying-open No. 13361/74 a method for simultaneously adjusting the pH to 3.55-3.80, the acidity as lactic acid to 0.35-0.50 and the milk-solids-not-fat content to 1.35-2.65%. However, according to this method milky white can be obtained, but the precipitation which is not recognized immediately after the production becomes recognized considerably within a short period of preservation. Moreover, in Japanese Patent Application Publication No. 20508/74, there is disclosed a method in which the fermented milk is made so as to have a pH below 3.5 and heated at a temperature above 60.degree. C., and thereafter microorganism and denatured protein are removed by means of a centrifugal separator and the like thereby producing a stable fermented milk drink. According to this method, on the contrary to said Japanese Patent Laying-open No. 13361/74, a drink which does not cause the precipitation for a long period can be obtained, but it has a very weak milky white and it becomes rather a transparent drink.
From the study of the characteristics of the prior art such as described above it has been found that the present status is such that in these prior art methods, although the consideration is paid for making the dispersion property of milk protein better, the former method retains the milky white appearance brought about by the milk protein particles produced by the passing-across of the isoelectric point as the milky white appearance of the final product at the sacrifice of stable dispersion of milk protein, while the latter method retains stability of the milk protein particles produced by the passing-across of the isoelectric point at the sacrifice of the production of a sufficient milky white appearance. Moreover, it has also been found that the size of the milk protein particles produced by the passing-across of the isoelectric point covers a wide range from smaller one to considerably large one. Accordingly, the prior art methods have a drawback that if it is wanted to maintain the milky white, the larger particles are precipitated, and on the other hand, if it is wanted to prevent the precipitation, the particles are made too fine or the larger particles are removed outside the system thereby resulting in disappearance of the milky white.